60 replies to this topic

[BYoesle]

I have noted there are quite a few images posted that are not on-band. For those new to H alpha observing, knowing the subtle differences between being on-band or slightly off-band can be difficult. This is made even worse when there is a lack of activity – few filaments or bright plage in which to judge by.

 

Here’s a little trick I discovered several years ago that has proven to work well, and is especially useful when there is a low amount of solar activity.

 

Remove the eyepiece, keeping the blocking filter in place. You will see some variation in brightness of the etalon across its diameter, which can be both a tuning and etalon quality indicator. This is equivalent to the collimated beam test using the Sun.

 

These darker/grey areas increase with tilting the etalon further off-band, and it almost "blacks out" when far off-band. This dimming gets reduced when tilted closer on-band. Uniformity also seems to improve as the etalon thermally stabilizes. This test shows the overall uniformity of the etalon gap, as well as providing the brightest image when on-band. You will also notice the image changes over time as the etalon thermally stabilizes.

 

 

I have used this test as a way to finely adjust my tilt-tuned  etalons, and knowing it's best when you get the most uniform brightness across the image as possible. It will also work with pressure tuned etalons. Try and observe this effect the next time you're out with one of the air-spaced etalons:

 

 

The simulated appearance of air-spaced etalon with improving tilt-tuning and thermal stabilization. Your goal is to get the least amount of brightness variation and the brightest possible image.

[Jason Irby]

Awesome. Thanks.

I have felt my tuning was pure guesswork. This at least gives me another tool to sanity check.

Goes for both SS and after I add the DS?

Edited by Jason Irby, 23 January 2021 - 07:27 PM.

[TOMDEY]

Interesting, and makes sense! Thinking further on it, the best placement for the pupil of one's eye would be at the center of the image of the sun, so that the integrated flux averages out the Doppler shifts and avoids the E/W rotational differential Doppler... but those effects are pretty small. I'll try that next time I'm waiting for the scope to stabilize; Thanks!    Tom

[gustavo_sanchez]

Interesting.

I usually do imaging. What I use to determine if I’m on band or not with my single pressure tuned etalon is to check the histogram level. The image gets dimmer and more contrasty when approaching on-band, so the histogram moves to the left. If I tune past the on-band position, the histogram will start to move to the right again. Thus, on-band is achieved with the lowest histogram level for a correctly exposed image.

I wonder, how the approach I described above correlates with your visual approach? Not disagreeing at all (I don’t consider myself an expert in any sense anyway), just would like to understand the whole picture.

[davidpitre]

I’ve also always felt I was guessing.
Thanks for sharing Bob.
I suppose with a double stack, one could do each separately, then put them together.

Or would it be better to tune the first, and then tune the second with them stacked?

Edited by davidpitre, 23 January 2021 - 10:29 PM.

[MalVeauX]

Great tip Bob, thanks! I will try this on my next session!

 

I've tuned my SM60 etalons visually so often that then I go to image and they're clearly not both on band together with a double limb. Even with fairly high contrast on plage and filaments, it's so easy to be slightly off band and still have that double limb. So if this helps get closer, then awesome!

 

Very best,

Edited by MalVeauX, 23 January 2021 - 09:37 PM.

[Gray]

Thank you Bob!

[BYoesle]

Thanks everyone, and I hope I haven't led you down a rabbit hole. I'm just relating what I have observed with my tilt-tuned etalons which have a CWL close to being on-band and needing very little tilt to get on-band. YMMV...

 

I usually do imaging. What I use to determine if I’m on band or not with my single pressure tuned etalon is to check the histogram level. The image gets dimmer and more contrasty when approaching on-band, so the histogram moves to the left. If I tune past the on-band position, the histogram will start to move to the right again. Thus, on-band is achieved with the lowest histogram level for a correctly exposed image.

I wonder, how the approach I described above correlates with your visual approach? Not disagreeing at all (I don’t consider myself an expert in any sense anyway), just would like to understand the whole picture.

 

Hi Gustavo,

 

Well, first, I'm no expert either. It's what I have observed and used numerous times, but it does seem a bit at odd's with your histogram method. But from your description, and my limited experiments with using the histogram for focusing, I might postulate the histogram going from bright to dark may be similar to focusing. It might be as shown below:

 

 

Here one can see the solar spectrum features (this is a conceptual diagram only BTW ;-) The H alpha photosphere absorption line contains the H alpha emission line of the chromosphere. The etalon produces narrow etalon peaks, all but one of which is blocked by the blocking filter. This etalon peak wavelength is tunable via tilting or changing the (gas/mechanical) pressure.

 

My guess is that when the pressure tuned etalon shows a bright image, the etalon peak is somewhere between A and B (or similarly the opposite side of the emission line). This is between the blocking filter passband limit and the H alpha absorption line (mostly if not all photosphere). As the etalon gets closer to being on band, the histogram becomes darker between B and C as you enter the absorbtion line. If you're averaging back and forth between position B-C on one side of the etalon peak and the same relative positions on the other side of the H alpha emission line, you should end up at position D being on-band. But again this is just theory until someone can verify this scenario.

 

The other possibility is that in starting out in the wings of the etalon, you have more parasitic continuum leaking through in the B to D region, and it isn't until you get on-band that the continuum is reduced or eliminated and only the chromosphere at D is seen, which is dimmer than the parasitic continuum leakage. 

 

 

What I think is happening with my tilt-tuned etalon - which has a far-less ability to tune off-band - is that when I observe the essentially collimated light from the Sun via the etalon peak, I'm somewhere in the region of the filter transmission profile "wings," B-Cish to D, and that with the proper tilt, the image becomes brightest on-band at D.

 

That's my theory anyway. Hopefully some of you can verify what you find and how it fits with this postulation.

 

I suppose with a double stack, one could do each separately, then put them together.

Or would it be better to tune the first, and then tune the second with them stacked?

 

Hi David,

 

I think it's usually easier to tune the primary etalon first, then add the secondary DS etalon, and go from there with the tilt and/or pressure tuning. The DS etalon will usually require some tilting, which changes the tuning it would have had if tuned separately. OTOH, once you tilt the secondary etalon to remove ghost reflections, you can remove it and - being sure to not change the tilt setting - tune it separately, and then reinstall it. This should result in the optimum DS tuning.

Edited by BYoesle, 25 January 2021 - 11:41 AM.

[UniversalMaster]

Does this also work with a Quark?

Edited by UniversalMaster, 25 January 2021 - 06:38 AM.

[wargrafix]

Ladies and gentlemen! We have ourselves a winner!

 

I tried his method, and I was on band withing minutes! Dannnngggggggggggggggg

[BYoesle]

Does this also work with a Quark?

 

Tuning with these filters is done by heating, and relatively slowly. Therefore it would have to be iterations of various temperature settings you're checking.

 

It should be a way to determine overall etalon uniformity once the filter has stabilized in temperature.

Edited by BYoesle, 25 January 2021 - 12:58 PM.

[StevenYood]

Brilliant and much appreciated.

[ilan_shapira]

Thank you very much for this. I was on the guessing side and judged by the image itself - the wrong method for my lack of experience.

Will give it a go as soon as the conditions allow.

[peterm]

Indeed, thank you. Having something, anything to use as a reference point is very helpful. I had thought "well that looks about right" was good enough, but clearly had no idea of what or where "about right" is.  Thanks again.

[davidpitre]

 

 

I think it's usually easier to tune the primary etalon first, then add the secondary DS etalon, and go from there with the tilt and/or pressure tuning. The DS etalon will usually require some tilting, which changes the tuning it would have had if tuned separately. OTOH, once you tilt the secondary etalon to remove ghost reflections, you can remove it and - being sure to not change the tilt setting - tune it separately, and then reinstall it. This should result in the optimum DS tuning.

Im a little confused. The tilt adjustment is the tune adjustment. There is only a single adjustment possible. Correct? Tilting  of the double stack etalon is a compromise of having it on band with the secondary reflection just out of the FOV.  Si?

[SloMoe]

I think Bob just invented the napkin,,,,,,,,,,, 

[BYoesle]

Im a little confused. The tilt adjustment is the tune adjustment. There is only a single adjustment possible. Correct? Tilting  of the double stack etalon is a compromise of having it on band with the secondary reflection just out of the FOV.  Si?

Si indeed! With the original Coronado Tucson SM etalons that is correct.   But you don't really need the ghost reflection to be out of the field of view - just off the main disc image is usually better - especially if this puts the etalon closer to being on-band. This will give the highest possible double stacking contrast. Indeed, theoretically the ideal DS etalon would require no tilt and line up perfectly with the main etalon image and overlap it.

 

The original Coronado Tucson etalons were generally very close to having their CWL on-band and needed little tilt for tuning, and when used for DS etalons with a little tilt they would be close to on-band and remove the ghost image(s) from the main disc. Extra tilt to remove ghost reflections would take them a little off-band, but not much. There is always some variation in the etalon-to-etalon CWL, and with double stacking, you'd use the etalon that needed more tilt to be on-band as the secondary DS etalon due to this essentially built-in tilt requirement.

 

Latter Meade Coronado SM etalons have a CWL that's deliberately further off-band, and require additional tilt to come on band - and thereby also removing the ghost reflections. Lunt front mounted DS etalons also seem to be in this category.

 

The Coronado SMII and III etalons also seem to be tuned a bit further off-band. After tilting to remove ghosts you can use the RichView central spacer pressure to get them on-band (or at least part of them).

 

As with everything, the devil is in the details and quality of the execution...

Edited by BYoesle, 26 January 2021 - 01:15 PM.

[SloMoe]

It's little tips like Bob's that I miss from starparties, dang covid,,,,,,,

[bigdob24]

Bob

As I just got my scope back from Lunt , I can’t wait to tune it to my satisfaction and then give your method a try and see what it looks like in SS and DS and adjust if needed. 

Also you mention CWL , I’m having a moment?

[BYoesle]

Hi BD,

 

CWL: Central WaveLength of an etalon transmission peak. Generally refers to the wavelength of interest when the etalon is normal (perpendicular) to the incoming collimated rays. In solar filters this transmission peak is isolated from the adjoining etalon peaks via the wide bandpass blocking filter system.

 

For the typical FP etalon intended for solar observation of the H alpha 656.28 nm chromosphere emission, the CWL is "tuned" in manufacture to be slightly above (tilt tuning, RichView tuning) or below (air pressure tuning) this wavelength to allow for tuning the etalon. Tilting etalons or applying mechanical RichView pressure blue-shift the etalon to be on-band. Lunt pressure tuned etalons are red-shifted with additional air pressure to be on band. These changes in CWL are needed to compensate for barometric pressure changes and to a lesser extent spacer substrate and air temperature changes which can shift the CWL.

 

For the solid etalons, they are made to have an on-band CWL when at a specified higher than ambient temperature (e.g. mica substrate thickness), which should remain stable above any usually encountered ambient temperature. If the temperature is below that needed to be on-band the filter is blue shifted, and if higher, red shifted. Since they are solid spaced etalons, they are impervious to normal barometric pressure changes.

Edited by BYoesle, 26 January 2021 - 03:14 PM.

[ATL Gator]

Hi all -- 

 

Newbie to H-alpha here. I'm looking forward to trying Bob's tuning trick if clear skies ever return to Atlanta. In the meantime, I'm wondering if anyone can tell me whether I'm on-band based on the attached image:

 

 

This shot was taken on Jan. 23 with a ZWO ASI178MM camera connected to a double-stacked Lunt LS50THa. Seeing was below average but not terrible. The image was stacked using AS!3 (no flats) and sharpened with ImPPG. I adjusted the tone using Photoshop and converted it to a smaller jpeg file in order to post on Cloudy Nights, but have otherwise left the image as-is -- I haven't done any additional sharpening or tried to correct the gradient. As you can see, there's a faint ghost image just to the right of the sun (I think it's in the correct position based on advice I've seen in this forum, but please let me know if I'm mistaken!).

 

To my untrained eye, it looks like the resolution is sharpest in the center of the disk, then degrades toward the limb, especially on the right side. Is this what I should expect using the gear I have, or does it point to an issue with my tuning? I realize there's a learning curve here -- just trying to determine whether I'm on the right track!

 

Best,

 

Scott

 

   

[rigel123]

It looks like the “sweet spot” is just a bit more to the left in your FOV so not totally centered. You can try to adjust where the sweet spot is or move the disk in the FOV. The ideal would be to adjust your tuning so the sweet spot is centered in your FOV.

[ATL Gator]

It looks like the “sweet spot” is just a bit more to the left in your FOV so not totally centered. You can try to adjust where the sweet spot is or move the disk in the FOV. The ideal would be to adjust your tuning so the sweet spot is centered in your FOV.

Thanks for the tip -- I will try that hopefully later this week. 

[Stickman]

    Bob's trick is a home run.  The clouds finally parted this morning and seeing was pretty good (1.33 arc sec.) with temperatures between 34 - 37 degrees.  I spent a few minutes adjusting my ethlons per his suggestion and the view was superb.  Awesome detail on the disk and limb, with a large prom I could see changing shape over my observation time. 

    I did notice after 10 to 20 minutes my scope would start drifting off band and I would have to readjust the ethlons.  I have a Lunt DS 100mm pressure tuned scope and am wondering if I need to replace the O rings in the ethlons (the scope is new to me, but is ~4 1/2 years old) or if the band drift is due to something else I've not considered?  Thanks for any ideas/input. 

 

     Rick

[dhkaiser]

     wondering if I need to replace the O rings in the ethlons (the scope is new to me, but is ~4 1/2 years old) or if the band drift is due to something else I've not considered?  Thanks for any ideas/input. 

 

     Rick

 

Good chance it was the colder temp affecting your blocking filter.  I have found heating the BF helps in cold temps.  Lunt has a BF heater.  Some people use dew heaters.